Field of the Invention
This invention relates to a lens apparatus and to a method of controlling the operation thereof.
Description of the Related Art
A high-performance lens apparatus such as a television lens incorporates a magnifying lens, which is referred to as an extender lens, and a master lens for adjusting in-focus position (focal point). The extender lens is capable of changing imaging magnification to a plurality of magnifications such as magnification by a factor of 1 or 2. The master lens is for adjusting flange back.
There is a system that adjusts the position of a focusing lens in accordance with zoom position based upon the corresponding relationship between zoom-end positions and focal point (Patent Document 1), a system that applies a focusing-position correction in accordance with zoom position (Patent Document 2), and a system that detects position information from a potentiometer, performs a correction computation using a control circuit and decides the master-lens position (Patent Document 3).
Patent Document 1: Japanese Patent Application Laid-Open No. 2004-233892
Patent Document 2: Japanese Patent Application Laid-Open No. 2003-57528
Patent Document 3: Japanese Patent Application Laid-Open No. 10-186209
Since an extender lens has its in-focus position adjusted at the time of shipment from the factory, there will be no shift in the in-focus position even if the imaging magnification is changed. Although there are instances where the lens cannot be maintained in an ideal state and the in-focus position may shift owing to the effects of the environment and the passage of time and the like, the user (cameraman) cannot tell that the in-focus position has shifted. As set forth above, Patent Document 1 adjusts the position of a focusing lens in accordance with zoom position, Patent Document 2 corrects focusing position in accordance with zoom position, and Patent Document 3 detects position information from a potentiometer and decides the position of the master lens, but none of these take into consideration the above-mentioned case where a shift in in-focus position occurs. Although the shift in in-focus position is caused by some failure, the user cannot determine whether the lens apparatus has malfunctioned.